We present a simple method to calibrate optical tweezers by using only videomicroscopy to measure the position autocorrelation function of the trapped bead in the potential well of the tweezers. To accomplish this task, we use a high-speed camera with ∼500 fps (frames per second), which provides a precise measurement of the relaxation time of the bead Brownian fluctuations. We also study the variation of the trap stiffness as a function of some parameters of interest such as the laser power and the distance from the bead center to the microscope coverslip, showing that the presented method returns precise results.